1. Field of the Invention
The present invention relates generally to energy storage devices and more particularly to a high energy flywheel design.
2. Description of the Prior Art
The use of flywheels for high energy storage in such applications as hybrid electric vehicles, earth orbiting satellites, military weapons, power utilities, load leveling, space stations and uninterruptable power supplies for computer facilities and electronic manufacturing has long been recognized and new applications are being realized every day. Many of these new applications require greater energy storage capacity and higher energy density (the ratio of the energy stored in a device to the device weight) than is currently available. Flywheel developers have been unable to produce a high energy storing flywheel that is small and light enough to satisfy the demands of the highest potential markets because of the stresses encountered at high speeds. For example, NASA and DOD projects as well as hybrid vehicle applications require a very light weight and very small flywheel.
Because the kinetic energy of a rotating body is equal to 0.5I.omega..sup.2 (in which I is the body's moment of inertia and (o) is the angular velocity), efforts to increase the stored energy and energy density of flywheels have concentrated on increasing the angular velocity. However, as angular velocity increases, radial expansion in the flywheel components also increases. Because the components expand at different rates, stress is placed on the component interfaces, as for example between the hub and rotor or rim. In addition, flywheel imbalances, caused by component misalignment and eccentricity, can attenuate potentially destructive resonances and place high loads on components such as shaft bearings.
Attempts to improve the integrity of component interfaces have focused on increasing the stiffness or strength of the attachment mechanism by such expedients as adding mechanical fasteners, increasing the size of component cross sections and incorporating compression pre-loads.
Some flywheel designers have included compliant structures. U.S. Pat. No. 5,124,605 addresses the need for the rim and hub to grow equally in order to eliminate the transfer of outer radial stresses to the hub axis. This is because the inner and outer surfaces of the rim expand outwardly during high speed rotation, the hub must expand at least as rapidly.
The '605 patent suggests that this can be accomplished if the modulus of elasticity of the hub is less than that of the rim. In the patent, a plurality of tube assemblies are positioned coupling the hub and rim. The tube assemblies are pre-compressed or pre-loaded within the flywheel but are purported to maintain dynamic stability at high rotational velocities.
U.S. Pat. No. 4,821,599 couples a rim to a central portion which includes at least one substantially circular, dished member. The dished member is curved such that increasing rotational speed tends to elastically deform the dish member, thereby straightening the curve and increasing the diameter of the dish member. Preferably the dish member includes three integrally formed annular portions of which at least two are curved and positioned on each side of the third portion. In a preferred embodiment the central portion comprises at least two coaxial dished members.
U.S. Pat. No. 4,058,024 discloses an inertial energy rotor having a plurality of independent concentric rotor rings rotatable about a vertical axis. A spacer ring connects each outer rotor ring to its adjacent inner rotor ring and is constructed of substantially rigid material. A spacer ring has a cylindrical configuration and a plurality of slots which alternately extend from opposing axial ends of the ring toward the opposite end of the ring.
U.S. Pat. No. 4,991,462 shows an ultracentrifuge rotor with a hub having radially outwardly extending curved spokes. The outer ends of the spokes are received in a groove defined on the inner peripheral surface of an annular rim. As the rotor rotates, the disparity in physical properties between the hub and the rim as well as the flattening of the curvature of the spokes causes the hub to grow to an extent at least equal to that of the growth of the rim.
U.S. Pat. No. 5,566,588 discloses a flywheel energy storage system having a flywheel rotor which provides high energy storage capacity and has an outer, primarily cylindrical body having conically tapered end sections, a conical hub section attached to the outer body and a relatively short inner cylinder. The cylinder connects the shaft to the inner portion of the conical hub section. In the preferred embodiment, the individual components are predominantly fabricated from filament wound fiber composites which allow material choices. The inner portion of the inner cylinder can be a slotted aluminum cylinder. Thus, this patent teaches a rotor including a cylindrical outer portion for storing most of the energy and a hub portion attaching the outer portion to the shaft. In the exemplary case, the hub portion includes a thin wall conical member which can be attached to the outer cylinder portion at the outer extremity of the hub portion and an inner cylindrical member of relatively short axial extent upon which the conical member is wound.
My prior co-pending application, Ser. No. 08/612,711, now U.S. Pat. No. 5,732,603 discloses a flywheel hub which includes an annular hoop and a pair of compliant diaphragms which are connected to the hoop. The diaphragms have apertures which receive a flywheel shaft and the hoop is sized to receive an annular rim. The diaphragms achieve compliance principally with a serpentine, radial cross section. The hub is preferably fabricated of a fiber and matrix composite. Because the compliant hub tends to match the radial expansion of the adjoining flywheel components, the integrity between the components is enhanced. In addition, the hub permits the rim to position its center of mass on the rotational axis to reduce rim vibrations. Axial spacing of the diaphragms provides moment stiffness to limit and control the rim deflection caused, by for example, gyroscopic precession forces.
Some single core design hubs can be found but are deficient in that they exhibit axial motions parallel to the shaft that are much larger in magnitude than the expansion matching achieved by the design.
As a result, conical disks have not been widely utilized in high speed flywheel designs. Alternatively, axially stable "flexure" hub approaches achieve expansion matching through intrinsic "stretch" of the designs, but always at the sacrifice of stiffness necessary that would promote rotordynamic stability. This high flexibility coupled with the low mass of flexure hub approaches make these design approaches incapable of producing "compression locking" at the hub to rotor interface. Consequently, these design approaches fail to address the catastrophic radial tensile stresses that are the limiting feature of all filament wound composite rotor designs.